An upper roller among three rollers of an existing pyramidal, symmetrical pipe bending machine is generally mounted on a slider that is vertically movable by a hydraulic cylinder. The hydraulic circuit ensuring that the movable roller-holder slider is linearly driven comprises a hydraulic cylinder whose rod is connected to the roller-holder slider. The hydraulic cylinder has an upper chamber and a lower chamber, both chambers communicating with respective ducts of pressurized fluid that is feed from a reservoir by a pump. A three-position four-way valve and a check valve operate on both ducts. These valves, as well as the pump, are controlled by an electronic control unit.
The roller-holder slider travels downward in a primary motion to a bending position and upward in a return motion to a rest position.
As known, pipes or other section bars are bent among the three rollers of a pipe bending machine through an operation including one pass or more with a result of the desired deformation. In order to obtain this deformation, specially when one workpiece or more has to be bent exactly with the same bending radius, one must keep for every workpiece the same position of the movable roller with respect to the fixed rollers, wherein the bending operation is performed.
The valves in the hydraulic circuit, controlled by the electronic control unit, do not ensure an exact positioning of the hydraulic cylinder rod at the required vertical position in the various passes. This is owing to a number of forces in action, among which the frictional force in the downward motion, the resistance of the material of the workpiece to be bent, the active forces of the piston of the hydraulic cylinder, the seal deflecting force, the elastic force of the piston ring, the counteracting differential thermodynamic force, the time constant of the control response of the electromagnetic valve, the viscosity of the hydraulic fluid and its non homogeneity due to the presence of air having a coefficient of compressibility different from that one of the hydraulic fluid.
In order to remedy the imprecision of the actual point of halting the movable roller-holder slider, mechanical stops have been used in the past that can ensure, without ascertainable error, the exact position of the movable roller in which a respective bending pass has to be performed.
However, to set the points of halting by mechanical stops is a troublesome operation, that involves an added effort and waste of time, in particular when deformations with great radiuses of curvature have to be performed.
Further, in the past, in order to solve the problem, complicate hydraulic circuits provided with proportional valves involving increased costs have been used.